Histo: Skeletal, Cardiac, and Smooth Muscles (smooth not complete) Flashcards
What are elongated muscle cells called and how are they oriented?
- Myocytes
- Parallel to each other in organized bundles
What is muscle cytoplasm called and what is it occupied by?
- Sarcoplasm
- Actin and myosin myofilaments
What tissue type?
- skeletal
- look for striations and cells on outside
What tissue type?
- cardiac
- look for circular dots with lots of space
What tissue type?
- smooth
- 2 different sheets
Skeletal muscle activity fastest
strong
quick
discontinuous
voluntary contraction
striated
Cardiac muscle activity
strong
quick
continuous
involuntary contraction
striated
speed can be modulated
Smooth muscle activity
weak
slow
involuntary contraction
no striations
speed can be modulated
Skeletal muscle general features
- made up of dense fibers that are organized and have high metabolic activity
- syncytium (share a cytoplasm)
Connective tissue investment layers
(outermost) Epimysium -> Perimysium -> Endomysium -> muscle fiber
Epimysium
- outer layer
- fuses and becomes tendon which moves bone
Perimysium
- invests fascicles (groups of myocytes)
Endomysium
- CT element that surrounds individual myocytes/fibers
- thin and small
- contains capillaries
Capillary network
- exists in endomysium
What is essential for the force of transduction of muscle fibers/bundles?
CT surrounding mus fiber
Classification of skeletal muscle fibers is based on 3 things:
- contractile speed: rate of contraction and relaxation
- enzymatic velocity: rate of ATP breakdown by myosin ATPase during contraction
- metabolic profile: capacity for ATP production by oxidative phosphorylation or glycolysis
T1 Muscle Fiber
- Slow oxidative fibers
- red
- slow twitch / slow myosin ATPase
- Many mitochindria and glycogen
What type of athlete has T1 muscle fibers?
- Long distance runners
- Meant for long, slow/sustained contraction needed to maintain posture
- Fatigue resistant
What type of athlete has T2a muscle fibers?
- 400-800m Sprinters, hockey players
- Fatigue resistant
What type of athlete has T2b muscle fibers?
- Short-distance sprinters, weight lifters
- Meant for rapid contractions and fine, precise movements
- Fatigue prone
T2a Muscle Fiber
- Fast oxidative, glycolytic fibers
- medium pink color
- fast twitch
- high glycogen mitochondria and myoglobin content
*Capable of anaerobic metabolism
T2b Muscle Fiber
- Fast glycolytic fibers
- white/light pink color
- fast twitch
- less mitochondria and myoglobin content (less metabolically active)
- high glycogen and anaerobic activity
*Myosin ATPase fastest of all fibers
*Last fibers to be recruited by nervous system
What is the structural and functional subunit of the myofiber?
Myofibrils
What is a myofibril?
- short, linear repeating unit of a sarcomere
- principle sarcoplasmic component of the muscle fiber
What are the functional contractile apparatus of skeletal and cardiac muscles?
Sarcomeres (bounded by z-lines)
What 2 types of myofilaments are myofibrils composed of?
-thin filaments (actin)
-thick filaments (myosin II)
Add labeled Z,I,M
Add labeled Z,I,M
What is a sarcomere bounded and centered by?
Bounded: Z, I
Centered: A, H, M
Actin:
- principal contractile protein of thin myofilaments
- pair of helically entwined polymers (f-actin) of G-actin molecule
- associated with troponin & tropomyosin
What binds actin?
TnI
What binds Ca+?
TnC
What binds tropomyosin?
TnT
Myosin II:
- composed of 2 polypeptide heavy chains, each with a “head” and rod-like “tail” portion
- 4 light chains associated with the head of heavy chain
- head has ATPas activity and actin binding site
- the projecting globular heads form the cross- bridges between the thick & thin filament
Titin
- anchors myosin filaments in the Z lines
- very large, has elasticity
a-actinin
- short
- major component of Z line
- anchors actin to Z line
Nebulin
- an inelastic protein
- assists in stabilizing a-actinin/actin interactions at z line
Tropomodulin
- “caps” actin to regulate its length at its free end
Myomesin
- myosin binding protein
- holds myosin in register at the M line
C protein
- myosin binding protein
- lies adjacent to myomesin
- holds myosin in register at the M line
What makes up the M-line in chromatography
Myomesin and C protein
Sarcomere labeled
Do actin and myosin interact at rest?
NO
What band is not visible during muscle contraction?
I band
What occurs during muscle contraction?
Thin (A) filaments slide past thick (M) filaments, resulting in greater penetration into A bands
Costamere
Connects the sarcomere of the myocyte to the cell membrane
Dystrophin
Links actin to laminin in the basal lamina, is dysfunctional in muscular dystrophy
Desmin
Surrounds sarcomere at the Z-line attaching them to the sarcolemma
Sarcolemma
- cell membrane of muscle fiber
- invested with a basal lamina
Sarcoplasm
- cytoplasm of a muscle fiber
- many mitochondria
Sarcoplasmic reticulum (SR)
- special form of smooth er in muscles
- sequesters calcium during contractions
- rich in calsequestrin (binds calcium)
- triad is functional unit
What is a triad composed of?
- 1 T-tubule (has voltage sensitive proteins to control depolarization of sarcolemma)
- 2 Terminal cisternae (reservoir for calcium w/ voltage sensitive calcium channels)
Triad image
What is a motor unit?
A single alpha motor neuron/axon
and the skeletal muscle fibers it innervates
Properties of the Motor Unit:
- all or none law (either muscle cell contracts or it does not)
- muscle fibers of the same motor unit contract in unison
- the force generated by a muscle depends on the # of motor units contracting, the types of muscle fibers activated, and the rate and duration of motor neuron input
- fine control muscles: 1 motor axon can innervate 1-3 muscle fibers
- course control muscles: 1 axon can innervate 100s of muscles
What is the motor end plate?
A specialized synapse where a
motor axon terminates onto a myofiber
Where does communication occur in skeletal muscles?
motor end plate
Properties of the Motor End Plate:
- only 1 MEP per muscle fiber
- axons can branch and invaginate sarcolemma to form junctional folds (high SA)
- ACh is present on the presynaptic side of the junctional folds
- depolarization of the nerve causes
exocytosis, spilling ACh into the
synaptic cleft - this activates ACh receptors on the sarcolemma
- results in depolarization of the muscle
fiber and contraction occurs
EM of neuromuscular junction labeled
What are the sensory receptors in muscle?
- muscle spindle (fuse) -> gives rise to reflexes and movement
- golgi tendon organ (GTO) -> innervates the tendon
muscle spindle (fuse)
- detects changes in muscle length
- contain both sensory and motor fibers
- stretch reflex (stretch on muscle causes contraction)
golgi tendon organ (GTO)
- monitors tension developed in muscle
- afferent (sensory) fibers only
- prevents damage during excessive force generation (stimulation causes relaxation of muscle via inhibition of a-motorneurons)
Intrafusal muscle spindle
- innervated by gamma motor neurons (sensory and motor fibers)
- contract slightly, but NO force around a joint
- maintain LENGTH of the fuse
Extrafusal muscle spindle
- innervated by alpha motor neurons
- highly contracting fibers that generate force and supply the muscle with power
Muscle spindle image
Muscle spindle fuse image
Fuse image (longitudinal section)
GTO image (tendon of achilles)
How do skeletal muscles develop?
- fusion of myoblasts (single cells)
What tissue type do myoblasts come from?
mesoderm
What is myostatin?
a regulatory protein that negatively controls muscle size (growth and differentiation)
What is myoD?
a regulatory factor that activates the expression of muscle-specific genes
Development of skeletal muscle order
mesoderm -> myoblast -> multinucleated myotubes -> muscle fiber
What are satellite cells?
- muscle stem cells
- present in adults
- sit on edge of myocyte
- monitor damage
Satellite cell image
a = nucleus of satellite cell
b = collage
arrows = basal lamina
Satellite cell in EM
Role of satellite cells?
induce repair
Cardiac Muscle: Overview
- under involuntary nervous control (autonomic)
- no regenerative capacity, damaged heart muscle is replaced by CT scar
- fibers are NOT syncytial network, but rather individual cells
- contain striations and sarcomeres
Cardiac Muscle: Cellular Structure
- one or two centrally arranged nuclei
- cells are joined to each other by intercalated discs
- mitochondria and glycogen granules are enriched at the poles of the nucleus and referred to as sarcoplasmic cone
Cardiac Muscle: Ultrastructural Organization
Intercalated discs:
- junctional complexes that appear as dark transverse lines between muscle fibers
- 2 portions: lateral and transverse
Transverse portion:
- runs across fibers
- possesses 2 junctional specializations: fasciae adherentes and maculae adherentes (desmosomes)
*fasciae adherentes = anchor thin filaments
*macular adherentes = bind cells to prevent separation during contractions
Lateral portion (gap junctions):
- provide direct electrical communication between cells
Cardiac muscle organization labeled
Atrial granules
- found in atrial cardiac muscle cells
- contain the hormones atrial
natriuretic actor (ANF) and brain natriuretic factor (BNF) - both are diuretic hormones
Cardiac Muscle: Nervous Investment
- cardiac muscle fibers contract spontaneously
- initial stimulus for contraction is by sinoatrial node
- when stimulus reaches purkinje fibers which spread out the timing from the SA node
- innervation is modulatory: speeds up or slows down the intrinsic beat
T tubule system: cardiac vs skeletal
- cardiac muscle has larger t-tubules than skeletal muscle
- sarcoplasmic reticulum of cardiac muscle is poorly developed relative to the SR in skeletal muscle
- each cardiac cell T tubule is associated with one terminal cistern, forming dyads
instead of triads
What are purkinje fibers?
- modified cardiac muscle cells located
in the atrioventricular bundle of His - larger than ordinary cardiac muscle cells
- have a large number of gap junctions
- LOW myofibril content and HIGH glycogen content
EM of purkinje fiber
